1. Field of the Invention
The present invention relates to optical glasses containing bismuth oxide, more particularly to optical glasses having at least one of the properties of being free from opacification and being devitrified in a glass body under such conditions as press-molding, including precision presses and reheat presses as well as reheat tests thereof.
2. Related Art
In recent years equipments or instruments, equipped with optical systems, have been highly integrated and sophisticated, which leading to more and more demands for the optical systems in terms of high accuracy, lightweight and miniaturization, thus the optical systems have been mainly designed using aspheric lenses formed of high index/high dispersion glasses in order to reduce the number of lenses.
It is expensive and non-efficient in particular to produce aspheric lenses by way of grinding or polishing processes; therefore, the aspheric lenses are presently produced by lower cost mass-production processes without the grinding or polishing processes such that gobs or glass blocks are cut and grinded to form a preform material, the preform material is heated and softened then is pressure-molded by use of a mold having a highly precise surface.
In order to attain the object to mass-produce the aspheric lenses with lower cost, it is necessary to investigate various conditions so as to satisfy items (i) to (iii) below:
(i) the glass is free from devitrification i.e. maintains transparency under reheating conditions, for example, of reheating-pressing processes for softening gobs or glass blocks by heating thereof;
(ii) the glass has superior chemical durability such that particular control is unnecessary in handling thereof after the polishing step; and
(iii) the temperature at mold-pressing step is as low as possible, so that molds for the mold-pressing can be far from surface oxidation and thus be repeatedly usable (there exists a relation between upper temperatures at mold-pressing and transition temperatures; the progress of the surface oxidation may be slower as these temperatures being lower).
With respect to (i) described above, the glasses based on TiO2 or Nb2O5 containing SiO2 or B2O3 as a former tend to exhibit relatively higher transition temperatures or higher glass yield points. Accordingly, these glasses are inappropriate for mass production, since crystals are likely to deposit at reheating steps at producing aspheric lenses, which causing problems such as lowering of process yield.
On the other hand, Patent Literatures 1 and 2 disclose glasses based on P2O5 utilized as precision-press materials. These materials may be softened and press-molded at temperatures lower than those of conventional SiO2 glasses. However, these glasses still exhibit higher glass transition temperatures, so that the glasses react with surfaces of mold materials, consequently optical parts come to difficult to reproduce the surface accuracy at the transferred surfaces through the precision-molding processes, and also the surfaces of mold materials tend to be injured. Furthermore, these glasses are likely to cause the devitrification due to basic components of P2O5, TiO2, Nb2O5 or WO3 through the reheating step, and also are relatively difficult to undergo precision press-molding due to problems such as possible fusion with molds or their clacks.
In addition, Patent Literature 3 discloses a glass containing Bi2O3 as a basic component; however, the refractive index and the dispersion are insufficient and also the glass transition point is higher. Furthermore, there exist such problems as the glass tends to display considerable opacification or to color into black at the reheating step in producing processes of the aspheric lenses or at reheating tests corresponding to reheat presses.    Patent Literature 1: Japanese Unexamined Patent Publication No. 07-97234    Patent Literature 2: Japanese Unexamined Patent Publication No. 2002-173336    Patent Literature 3: Japanese Unexamined Patent Publication No. 09-20530
The present invention has been made in light of the objects described above; that is, the present invention provides optical glasses containing bismuth oxide in which the optical glasses are having at least one of the properties of being free from opacification and being devitrified within glass body at the reheating step in producing processes of the aspheric lenses or at the reheating tests corresponding to the reheat presses, and also optical glasses with superior chemical durability and free from coloring into black.